Field of the Invention
The present invention generally relates to semiconductor integrated circuit devices, and more particularly to a semiconductor integrated circuit device for a non-contact type IC card supplied with electricity from a read/write unit.
Electricity is supplied from the read/write unit to the non-contact type IC card in an electromagnetic coupling in which an antenna coil of the read/write unit and an antenna coil of the non-contact type IC card are electromagnetically coupled. In such a formation, electricity obtained in the non-contact type IC card greatly depends on a communication distance between the read/write unit and the non-contact type IC card and power supplied from the read/write unit.
In such a kind of non-contact type IC card, it is expected that the distance between the non-contact type IC card and the read/write unit changes during communication and thus electricity supplied to the non-contact type IC card changes abruptly. In this case, a fault may occur. For example, the non-contact type IC card may malfunction and data may be lost. A failure in communication may also occur.
It is thus required that an LSI device used to construct the non-contact type IC card performs the function of stabilizing the power supply voltage and avoiding a malfunction and communication error resulting from a variation in the power supply voltage, and loss of data due to such a malfunction.
It is an object of the present invention to provide a semiconductor integrated circuit device capable of operating a non-contact type IC in a stabilized fashion.
A more specific object of the present invention is to provide a semiconductor integrated circuit device for use in a non-contact type IC card capable of avoiding various problems resulting from a variation in electricity supplied to the IC card.
The above objects of the present invention are achieved by a semiconductor integrated circuit device for a non-contact type IC card equipped with a rectifying circuit rectifying a received signal and thus producing a power supply voltage, said device comprising: a shunt regulator which is connected between a power supply voltage and ground and controls a shunt resistance: and a control circuit which controls the shunt regulator so that: the shunt resistance gradually decreases when the power supply voltage becomes higher than an upper limit of a reference voltage range, and gradually increases when the power supply voltage becomes lower than a lower limit thereof: and the shunt resistance remains constant when the power supply voltage falls within the reference voltage range.